To downsize the whole of a sheet aligning device receiving discharged sheets by an aligning sheet receiver, aligning the sheets on the sheet receiver, and then discharging those onto a stack tray.

On a frame 30, a main guide rod 19 and a sub guide rod 20 are provided in parallel with each other. The guide rod slidably supports sliders 12, 14. Joggers 13, 15 are attached to both sliders. The jogger are respectively and integrally provided with the aligning sheet receivers 13A, 15A. The sheets discharged from a paper discharging roller by receiving both side edges with the sheet receivers, and aligned on the sheet receivers. The stack tray is provided under the sheet receivers. After aligning, an interval of a pair of the sheet receivers is opened to drop the sheets, and the dropped sheets are received by the stack tray.